In multi-transmitter communications networks, channel access techniques allow multiple transmitters connected to the same physical channel to share its transmission capacity. Various such channel access techniques are known in the art. For example, in second generation communications systems according to the Global System for Mobile communications (GSM) standard, Time Division Multiple Access (TDMA) techniques are utilized to divide a specific frequency channel into individual time slots assigned to individual transmitters. In third generation communications systems, Code Division Multiple Access (CDMA) techniques divide channel access in the signal space by employing a combination of spread spectrum operations and a special coding scheme in which each transmitter is assigned an individual code. The next advance in wireless communications systems considers Orthogonal Frequency Division Multiple Access (OFDMA) techniques to achieve still higher bit rates.
One major advantage of OFDMA over other channel access techniques is its robustness in the presence of multi-path signal propagation. On the other hand, the waveform of OFDMA signals exhibits envelope fluctuations resulting in a comparatively high Peak-to-Average Power Ratio (PAPR). The disadvantage of a high PAPR inherent to OFDMA is to a certain extent overcome by Single Carrier Frequency Division Multiple Access (SC-FDMA), which can be regarded as a modification of the OFDMA technique. The Third Generation Partnership Project (3GPP) is considering using both OFDMA and SC-FDMA in next generation communications systems currently standardized in the Long Term Evolution (LTE) project.
According to section 5 of the 3GPP Technical Specification TS 36.211 “Physical Channels and Modulation”, V8.7.0 of May 2009, SC-FDMA will be implemented in LTE user equipment for the uplink direction towards the LTE access network. OFDMA, on the other hand, will be used in the downlink direction from the LTE access network towards the user equipment (see section 6 of TS 36.211).
In TS 36.211, the smallest time-frequency unit for downlink transmission is denoted a resource element. The mapping between resource elements on the one hand and physical channels on the other is described by resource blocks. A resource block is defined as a pre-determined number of consecutive SC-FDMA or OFDM symbols in the time domain and a pre-determined number of consecutive subcarriers in the frequency domain. An LTE downlink subframe may be represented as a resource grid comprising several resource blocks destined for different users.
Certain resource elements at pre-defined positions in the downlink resource grid are reserved for synchronization and reference signalling. The Primary Synchronization Signal (P-SS), for example, occupies 72 resource elements in the centre of the downlink signal spectrum. It occurs every 5 ms and shares its spectrum with resource elements carrying data. The P-SS is used by LTE user equipment for initial frequency offset estimation.
User equipment derives all clocks and frequencies from an internal reference oscillator. During regular operation, the oscillator is controlled to match the centre frequency of the receiver to the centre frequency of the downlink signal. Upon its activation, however, the reference oscillator is uncontrolled so that the receiver will typically have a frequency offset relative to the downlink signal. As part of the user equipment synchronization procedure it is therefore necessary to derive, based on the P-SS, an estimation of the frequency offset for an initial adjustment of the reference oscillator.
According to section 6.11.1.1 of TS 36.211, the P-SS is generated based on a Zadoff-Chu sequence. Each Zadoff-Chu sequence comprises complex-valued symbols (also called samples) which, when modulated onto a radio carrier, give rise to an electro-magnetic signal of constant amplitude. Signals comprising cyclically shifted versions of a specific Zadoff-Chu sequence do not cross-correlate (i.e., remain orthogonal to each other) when recovered at a receiver, provided that the cyclical shift is greater than a specific threshold defined by propagation delay and multi-path delay spread. An electromagnetic signal carrying a Zadoff-Chu sequence thus has a CAZAC waveform.
While exhibiting excellent correlation properties, CAZAC sequences and in particular Zadoff-Chu sequences are insensitive to frequency offsets. This means that, if the received P-SS is correlated by LTE user equipment with the Zadoff-Chu sequence originally used to generate the P-SS, a correlation peak can occur even though there might exist a frequency offset between the LTE access network and the user equipment. Due to the frequency offset the correlation peak will be shifted along the time axis with respect to the timing of the LTE access network, and the actual time shift depends on the specific frequency offset. However, since the timing of the LTE access network is not known either to the user equipment, it is not possible to conclude on the frequency offset. This time-frequency ambiguity is a problem when having to determine the frequency offset.